Many types of printing systems include one or more printheads that have arrays of marking elements that are controlled to make marks of particular sizes, colors, etc. in particular locations on the print media in order to print the desired image. In some types of printing systems the array of marking elements extends across the width, and the image can be printed one line at a time. However, the cost of a printhead that includes a page-width array of marking elements is too high for some types of printing applications, so a carriage printing architecture is used.
In a carriage printing system (whether for desktop printers, large area plotters, etc.) the printhead or printheads are mounted on a carriage that is moved past the recording medium in a carriage scan direction as the marking elements are actuated to make a swath of dots. At the end of the swath, the carriage is stopped, printing is temporarily halted and the recording medium is advanced. Then another swath is printed, so that the image is formed swath by swath. In a carriage printer, the marking element arrays are typically disposed along an array direction that is substantially parallel to the media advance direction, and substantially perpendicular to the carriage scan direction.
Recording media, whether supplied as cut sheets or from continuous rolls of media, is typically advanced by a set of rollers driven by a motor. The amount of roller angular rotation is controlled by the printer controller. Angular rotation θ can be implemented by specifying a number of advance steps by a stepper motor, and/or θ can be monitored by use of a rotary encoder that is mounted coaxially with a feed roller, for example. The distance that media has been advanced is nominally Rθ, where R is the radius of the media advance roller that is coaxially mounted with the rotary encoder and where θ is measured in radians. However, there are a variety of sources of error in this nominal media advance distance. First of all, manufacturing variability or wear in rollers can result in a roller radius that is not exactly equal to R. Secondly, the distance of advance of the side of the paper on which marking will occur is actually (R+t)θ, where t is the thickness of the media being advanced. Media thickness can therefore affect the advance distance. Thirdly, there can be slippage between the media and the roller.
For the case of under feeding the media, media advance errors can result in dark streaks in the image because adjacent swaths of printed data partially overlap. For the case of over feeding the media, media advance errors can result in white streaks in the image because there is a gap between adjacent swaths of printed data. In addition, overfeeding and underfeeding also results in the overall image length being too long or too short. Especially for long images, even relatively small systematic error in media feed distance can result in problems in framing or tiling of images.
A variety of methods for correcting for media advance errors have previously been disclosed. U.S. Pat. No. 5,825,378 discloses printing a series of lines, where successive lines are separated by media advance steps. The line spacing can then be measured by rotating the sheet of media 90 degrees and measuring the distance between lines using an optical sensor that is mounted on the carriage. The actual distance between lines is compared to the nominal distance between lines and the error is used to correct the angular rotation that the roller is to be advanced for a given desired media advance. This method requires direct user intervention to rotate the media.
U.S. Pat. No. 6,137,592 discloses printing a test pattern using successively increasing or decreasing values of media feed. The user then selects the region of the test pattern having a minimum amount of light or dark streaking. The media advance selected can then be stored in memory as the new nominal advance distance. This method requires user intervention to select the best looking portion of the test image, and is susceptible to user error.
U.S. Pat. No. 7,210,758 discloses printing a test pattern including an on-off pattern such as a checkerboard and incrementing media feed values. At an optimal media feed, the dark patterns from a first pass will line up with the light patterns from a second pass, so that the pattern appears darkest (maximum optical density) for the optimal media feed. Examination of the printed pattern can be done automatically by measuring optical density of the pattern and identifying the optimal media feed value as corresponding to the region of the test pattern having the maximum optical density.
While the aforementioned methods are satisfactory for some applications, as customer expectations for improved image quality continue to increase, there is a need for a media feed calibration method that is even more precise and less susceptible to measurement error.